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Is night VMC still VMC if the underlying terrain is completely unlighted?
The whole point of visual meteorological conditions (VMC) is that there is sufficient visibility for a pilot to be able to safely fly by visual reference alone, without reference to instruments.  This, by necessity, requires a visibility of at least one to several miles, plus staying a good distance away from any clouds (the exact visibility and cloud-clearance limits depend on the jurisdiction, but all have at least some minimum requirements for these factors); so far, so good.
In many countries, however (including a lot of the big ones, such as the US, Canada, the UK, Australia, France, Germany, and New Zealand), VMC is still VMC even at night, and night VFR flight is perfectly legal (albeit with somewhat greater restrictions on who can fly VFR at night and the qualifications necessary thereto) if the visibility and cloud-clearance requirements are satisfied.1
The underlying assumption is presumably that pilots will still be able to avoid flying into the ground (in either controlled or uncontrolled manner), or each other, by reference to ground lighting and each others' anticollision lights, but this line of reasoning breaks down if you fly over unlighted terrain (of which the countries permitting night VFR flight have quite a lot), especially if it's a moonless night.  If it's well and truly nighttime, the moon is either a thin crescent or completely absent from the sky, and the terrain is devoid (or nearly so) of light sources, the ground will be almost completely invisible, with the only hint of its presence being its occlusion of the background stars, and it is difficult to imagine how one would be able to maintain terrain clearance (especially if one's aircraft lacks a radar altimeter).  Even worse would be if the stars (and moon, if present) were obscured (by, for instance, high clouds, or smoke from forest fires), in which case one would have no external visual reference at all;2 a non-instrument-rated pilot would find it essentially impossible to maintain control of the aircraft in these circumstances, just as if they were trapped inside a cloudbank during a daytime flight, and even a pilot who is proficient in instrument flight would have difficulty maintaining terrain clearance, especially in mountainous areas (which, inconveniently for our pilot, tend not to be densely populated, and, thus, are rather more likely to be unlighted than flatter terrain).
To eliminate this problem, does the definition of night VMC (in the US or any other jurisdiction) require that the terrain in the area be lighted sufficiently for it to be visible?

1: In many other countries, VFR flight is not permitted at night, and the issue therefore does not arise.
2: Unless you're low enough for your aircraft's lights to illuminate the ground, in which case you're already way in deep shit anyways.

There is an age-old procedure for dealing with engine failures at night.  You descend until you are about 200 ft AGL by your altimeter.  You turn on your landing light.  If you don't like what you see, you turn it off.

This FAA doc has some good/related info on the topic.

Interesting question.  In the United States, VMC just stands for Visual Meteorological Conditions, meaning that these weather conditions permit flight by Visual Flight Rules.  IMC or Instrument Meteorological Conditions are meteorological conditions where flight operations must be performed by sole reference to flight instruments.  Note the terminology: METEOROLOGICAL CONDITIONS are only in reference to weather.  It does NOT make a distinction between day and night operations.  Instrument Flight Rules do not make a distinction on this point, either.
You are quite right to think that dark night over featureless, uninhabited terrain or over water is instrument flying; but is is still considered flight under VMC.  While you can LEGALLY conduct a flight over these areas at night under VFR, as a CFII, I will advise you that this is not a reasonable way to operate an aircraft and possibly very unsafe too.  John F Kennedy Jr was killed doing this near Martha's Vineyard in July 1999.  I also recommend that non-instrument rated pilots not attempt to fly VFR in these conditions.  For instrument rated pilots, I strongly recommend filing IFR during cross country night flights.

The short answer is no, the definition of night VMC does not specifically require any defined degree of ground illumination to be present.  (at least in the US)
You raise some excellent concerns in your question, demonstrating a solid understanding of the principles of Operational Risk Management:  Identify the hazards, assess the risk, make decisions, implement controls, and monitor.
Following these steps prior to, and during night flight is crucial to safety for the reasons you describe.  As long as visibility is relatively clear and there is enough ground lighting to form a discernable horizon, flying on even the darkest nights can be done safely, and can actually be fun.  It is easier to pick out other aircraft visually, and roads and cities really highlight themselves.  In areas where illumination is sparse, situational awareness of your position and the highest terrain in your sector is crucial.
It is rare for anybody to truly need to fly VFR at night, so if weather, visibility, ground lighting, or any other conditions are concerning to you, either don’t fly, or file IFR if you are so rated.

I used to do it but as I've aged, and having experienced a mechanical engine failure in the past, you won't get me up in a single at night any more.  Just too much nagging ass-pucker effect.

It's very much worth noting the level of sensitivity of the human eye.
This sensitivity, in a healthy, well nourished individual who is not hypoxic, is such that it is impossible to become fully dark-adapted outdoors on a clear, moonless night, because starlight is too bright.
From my own experience at star parties, I can attest that as long as participants are careful about use of artificial light, after less than an hour in the dark it becomes possible to walk around without colliding with people or objects, identify objects visually, and so forth.  In a photographic darkroom, I've noted that light from a luminous clock dial, reflected from a light colored wall, becomes sufficient to locate developing trays visually, despite being much too dim to fog film with reasonable exposure times.
If the instrument lights can be sufficiently dimmed and the eyes adapted before leaving the region with ground lights, a pilot flying well below oxygen required altitude (or using oxygen correctly) ought to be able to see the ground well enough to avoid problems, although starlight will tend to be very "flat" -- coming as it does from the entire sky, the will be no real shadows to highlight terrain features and potential obstacles.  The limiting factor here might well be the airplane's own navigation and clearance lights -- which, for obvious reasons, can't be dimmed or shut off while in flight.

I can confirm this from quite a bit of experience flying over the middle of Nevada & Oregon, where ground lights are few and far between.  Actually, having just one or a few ground lights, like the outside lights at remote ranches (which can be seen from a considerable distance on a clear night) is more of a problem than no light, since you tend to fixate on the light and become disoriented.

"does the definition of night VMC (in the US or any other jurisdiction) require that the terrain in the area be lighted sufficiently for it to be visible?"
In the US, the answer is no. It is certainly possible, but can be mentally challenging.
When I did my night training for my private pilot license, we flew southeast to overfly Martha's Vineyard, then east to land at Nantucket; see the picture. (We actually did some takeoff and landings at Minuteman, 6B6, to start the flight as 9B1 didn't have lights).  Martha's Vineyard is not too many miles off the coast, and on a clear night there are plenty of lights in the surrounding area to pick out as ground references on the way there. Once we turned away from MVY tho, I remember the loss of horizon vividly after a few minutes - I recall the plane, my Cessna 150, 100 HP, 2 seater, with just enough instruments to be legal for IFR, no autopilot, picking up speed, by the sound of the wind picking up, and realized I was not straight & level anymore.  My instructor was saying "trust your instruments, trust your instruments", and I used them to level off and stop the descending turn I had gotten myself into. Then following them until the we could pick up the lights at Nantucket.  Coming back was similar, climbing up into the darkness, following the instruments for several miles until lights became visible again.
It's harder than you think, and even now, 20+ years later, I find myself pretty drained after an IFR flight, even a training/practice flight "under the hood", because it is a lot of mental work.  Single pilot IFR is said to be one of the toughest flying tasks there is.  Admittedly it gets easier with moving map GPS and autopilot, but you have to practice without those too in case something happens and you're stuck on just the older technology, or even less.

At least in the US, no, the terrain doesn't need to be visible for night VFR flight to be legal. However, what is legal and what is safe are often quite different.
Even in rather sparesely populated areas, there will usually be some amount of lighting on the ground, e.g. farm houses, cars, towns, etc. And once your eyes adjust to the dark, the moon may illuminate the ground between enough to see it. If the sky is mostly clear, you may be able to see a horizon where the stars stop.
On a cloudy night over completely barren land (or open water), it's all just black in every direction, so one must fly solely by reference to instruments. You can actually log this as "actual instrument" time even when flying VFR, which should be a clue that doing so is probably a very bad idea for someone without an instrument rating.

I kinda already figured it was a bad idea - that's why I asked the question in the first place!  :-P

Actually, the thing about VMC vs. IMC is not primarily so much to do with whether you can see the ground below or around you (you can be VFR-on-top and see nothing but clouds underneath you, you're still considered to be VMC), but rather how you go about avoiding other traffic.
In VMC, the primary traffic avoidance/deconfliction method is "see-and-avoid", and crews performing VFR operations are expected to handle that burden individually with no or minimal assistance. In IMC, traffic avoidance/deconfliction is more of a team effort (though the individual crews are still held primarily responsible for their own aircraft) between all participants by following specific procedures, regulations, ATC and TCAS instructions, and so on.
That's what makes night VFR over featureless and unlit terrain an even more dangerous proposition than regular night flying; you'll need to spend a lot of time heads-down looking at instruments, but you'll still be expected to handle traffic deconfliction on your own.

Except that when you're flying over featureless & unlit terrain, there is generally not a whole lot of traffic to deal with, and what there is can be seen much more readily than during the day, thanks to navigation lights.  Nor in most cases do you really spend much more time looking at instruments, at least in typical nighttime conditions.  I suppose you could have high cloud cover blocking all light from the moon & stars, over unlight terrain, but I would consider that instrument conditions.

However much or little traffic there is to contend with, if you're in VMC, you're expected to be able to see other traffic around you and be much more proactive in avoiding it than when you're operating in IMC. Whether you can see the terrain below or around you is not that relevant to determining VMC vs IMC. Nighttime over unlit terrain is not the only place you can run into this kind of viz issue. Fly around at ~4k feet in the middle of a summer afternoon in Florida and you'll likely be in ball of haze, with no real horizon, and only able to see ground if you look straight down. Still VMC

But as I said, it's much easier to see that other traffic at night.  As long as you can tell which way is up (and I've never personally encountered a case where I couldn't), you don't need to spend any more time looking at instruments than in the day.  Though I agree about daytime haze, since I learned to fly in the LA Basin.  Technically VFR, and you can see the ground, but real hard to see other traffic.


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What is the location of AC for typical modern airplanes?
Some sources state that AC must be located behind CG, so that AC is stable.
There is a source, which throughout the study of stability and control consider AC located ahead of CG and positive cambered tailplane, stating that such airplane will be stable.
I understand that there is not only one right decision and it's possible to design aircraft with AC located both behind CG and in front of.
The question is, what is typical arrangement of CG and AC for most modern large jet transport aircraft? (especially interested in b737 and a320)

The aerodynamic centre of a wing is different from the AC of the whole airplane (its called N.P).@Nikita

AC of the whole aircraft must be located behind CG, so that AC was naturally statically stable [in pitch, or more accurately, by angle of attack]. This pretty much follows from the definition of AC, and there is no way around it.
However, this is not the whole story.
First, it is possible to design an airplane with 'positive cambered tailplane'. As such, there is no requirement that the tailplane produced negative lift (for a classic design); there is merely a requirement that the tailplane had a lower lift coefficient than the wing.
Second, a statically unstable airframe can still be made stable by using computer control. From the point of view of the pilot, F-16 or Su-27 are stable, even though their airframes are not. In some marginal cases, when the aircraft has a lot of pitch damping, even average pilot can cope with mild instability.
As for airliners, all of them have AC behind CG, but I don't have specific numbers.

Are there airliners, which have AC of the wing located ahead of the CG? (mostly interested in typical airliners, not marginal cases)

@Nikita, AC of the wing alone matters little for the whole aircraft stability analysis. For classic airfoils, it may well be forward of CG (of the aircraft), but so what? The tail is there to fix it. But again, I don't have any particular examples at hand.

Now I got it. Thank you

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What are the speeds for absolute ceiling and coffin corner?

This picture is taken from Safety First (Airbus magazine). Could you please clarify some things:
1). Is "Absolute aerodynamic ceiling" = "Coffin corner"?
2). Have I correctly understood that there is no "Absolute Ceiling" (ROC=0) in this diagram?
And a question that doesn't relate to this particular picture. At absolute ceiling we can fly at only one speed. This is the speed which corresponds to intersection of power available and power required curves? And what is that speed at which flight at Coffin Corner is only possible?

The coffin corner is where the lines for minimum speed (which increases with altitude) and maximum speed (which decreases with altitude) cross. Some of the other lines show safety margins designed to prevent aircraft from ever actually flying in the coffin corner because, as the name implies, it's quite dangerous.

At absolute ceiling we can fly at only one speed. This is the speed
  which corresponds to intersection of power available and power
  required curves?

The "intersection of power available and power required" is anywhere along the left hand edge of the red cross-hatched area. The absolute ceiling is the intersection of the stall curve (left-most red line) with this power limit.

And what is that speed at which flight at Coffin Corner is only possible?

Coffin corner in that chart should be the intersection of the red stall line with the black MMO/VMO line.
I say 'should be' because the stall line is drawn with a maximum at the  "absolute aerodynamic ceiling" i.e. the plane stalls if it goes any faster or slower. Stalling when going faster must be due to increased Mach number.
In other words, the right hand side of the stall line is actually the maximum Mach number, which would make the "absolute aerodynamic ceiling" 'coffin corner'.
I suspect the confusion is that they wanted to draw MMO as a (mostly) straight line, and didn't think it mattered because the plane doesn't have the power to reach that region.

As you said: absolute ceiling is at the intersection of stall curve and red cross-hatched area. Coffin corner is at the intersection of red stall line with black MMO/VMO line. Both of them doesn't correspond to "Absolute aerodynamic ceiling" marked in the picture. What is then that "Absolute aerodynamic ceiling"?

Good point, I've reconsidered and changed by answer.

Now I got it. One more question. Is it correct that GD and CI=0 must meet each other at "absolute aerodynamic ceiling"?

Now you've got me - I have no idea what either of those lines mean!

This graph is illustrative for a single weight, with the exception of Vmo/Mmo and Vd/Md.

Is "Absolute aerodynamic ceiling" = "Coffin corner"?

For all intents and purposes, coffin corner is where the top barber pole and the bottom barber pole on the speed tapes collide. This occurs when the stall warning speed or buffet onset speed coincides with the maximum operating speed (i.e. Vmo/Mmo). So while the graph defines Absolute Aerodynamic Ceiling as Vs1g, it will be lower than that.
The "Aerodynamic Ceiling" as defined, on the other hand, is the minimum operating speed for the aircraft at altitude. It is the minimum speed at which you should fly at to have enough margin for maneuvering. If you fly below this speed, you will see the barber poles converge as you are making turns or pulling up.

Have I correctly understood that there is no "Absolute Ceiling" (ROC=0) in this diagram?

There is, and it's the "maxspeed in level flight". This would correspond to a rate of climb of 0 at maximum thrust.

At absolute ceiling we can fly at only one speed. This is the speed which corresponds to intersection of power available and power required curves? And what is that speed at which flight at Coffin Corner is only possible?

Not sure what you mean by the last part. The blue curve titled "Service Curve" is roughly the intersection of power available to power required, although with a climb margin of 300ft/min. This is a performance limitation and has technically nothing to do with coffin corner.

Service ceiling is usually described as maximum altitude at which ROC of 500 ft/min for jet aircraft can be obtained. Have I correctly understood that judging by this chart, Airbus reduced this value to 300 ft/min?

@Nikita I wasn't aware that there is a strict threshold. I've heard of 100ft/min as well.

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Why does stall speed decreases with increasing aspect ratio?
As I have found Cl(max) doesn't change with changing AR. Is it true?
Therefore stall speed which corresponds to Cl(max) and crit AOA must remain constant regardless of AR.
Where am I mistaken?

Are you holding the total wing area constant?

$c_{L_{max}}$ doesn't change with changing AR. Is it true?

No, it isn't. Since the area near the wingtips cannot produce a pressure difference as large as that on the center wing due to flow around the wingtips, the lift there is reduced. The larger the wingtip part is relative  to the whole wing, the lower the maximum lift will be.
Given the same wing area and mass, a wing with a higher aspect ratio will produce a lower stall speed. Of course, the assumption of an identical mass means that the lower aspect ratio wing has a poorer structural design.


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Are there any locations where I could launch a rockoon with minimal red tape?
I'm currently working on a research project on hypersonic control surfaces, and as part of it, I'd like to launch a rocket-assisted lifting body from a high-altitude balloon.
Are there any locations in the US where I could do this? The first thought that comes to mind is launching at sea, but recovering a small craft (~ 1 meter wingspan) from the open ocean seems like a logistical nightmare. Ideally, I would prefer to launch over land that can be accessed with relative ease by car or offroad vehicle.
At one point, I read about a section of airspace in Alaska with basically no regulation but I can't seem to find that information now.
Thanks!

I'm not aware of any places in North America that restrict the launching of rocket powered racoons...

@JohnK Launching a rocket-powered raccoon may constitute animal cruelty, something many jurisdictions have laws against.


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What's this biplane built by a 16-year old?
I was told that this plane was built by a 16-year old in the 90s. Ever since, I've been bent on finding the specifications, name, etc., of this airplane with no success.

I don't know about this being from the 90s, but it sure looks like the Laird Baby Biplane in the Florida Air Museum to me:

On display at Florida Air Museum, Lakeland Linder Regional Airport, Florida (wikimedia.org)
The identifying features that I observe to be common between this picture and other pictures of the aircraft are the shape of the nose, the details along the fuselage, the shape and placement of the struts, the tanks beneath the top wing and the shape of the engine protrusion. I would also note that the Wikipedia page indicates that the specimen in the museum is a replica that was built in the 90s (whereas the original is from 1912).

Yess!..its is Laird baby biplane


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What is decalage and how it affects airplane performance?
What is decalage in airplanes and how important is it in respect to  airplane performance?

This article from flying magazine has a nice definition (including some history)

Decalage — a French word meaning “shift” or “offset” — is, broadly
  speaking, a difference between the incidences of any two lifting
  surfaces. It was originally applied to the two wings of a biplane: In
  the usual arrangement, the upper wing was farther forward than the
  lower and had a larger (can’t get away from that term!) angle of
  incidence — called positive decalage — so that it stalled first,
  shifting the center of lift aft and providing an automatic nose-down
  moment for recovery.
In a monoplane, the term refers to the angles at which the wing and
  the stabilizer (or canard) are attached to the fuselage. For an
  airplane to be longitudinally stable, it must have positive
  aerodynamic decalage; roughly speaking, the forward surface must be at
  a greater angle of attack than the aft one. This principle applies to
  conventional airplanes and to canards alike.


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Is having the tailplane at the wings backwash a design flaw?

Most airliners have their tailplane below or above the backwash of the wings, so is placing the tail control surfaces directly in the backwash  of the wing a disadvantage?

Keep in mind that the wing‘s „backwash“ might manifest only a good deal lower than the wing‘s vertical position along the fuselage, depending on flight conditions - after all, the wing deflects airflow downward.

It's not a bug, but a feature! Since the downwash angle increases with angle of attack, the elevator sees a lower range of angles than the wing. This means that at wing stall it still operates several degrees below its own stall angle, so being in the downwash makes it more stall-proof. When the wing loses lift (post-stall) and the downwash is reduced, the resulting change in angle of attack pitches the plane down, so again the downwash makes the tail do the right thing.

Insightful.......this should be an answering not a comment @Peter Kämpf

I would say not really. The main impact would be on what incidence angle to set for the horizontal stabilizer because of the difference in local flow angle in or out of the downwash, and on the trim changes to expect if there are wing flaps, larger trim changes in the downwash, less if out.
For your ultralight, I would say the priority is simplicity and lightness, so just stick the horizontal tail at the most structurally convenient location, which is more or less where it is in the picture, like 90% of the other airplanes out there.
Size it based on the standard formula, make an educated guess (er... calculation) on the incidence angle to set, but make an attachment for the stab leading edge with multiple holes that allows you to change the incidence after flight testing.
And remember that when you test fly your ultralight you are effectively an Experimental Test Pilot taking the risks that go with that job description.

One consideration involves the "T" tail design. During wing stall, the Horizontal stabilizer can be blanked by the wake of the stalling wing, rendering the Horizontal stabilizer almost completely ineffective for stall recovery. This condition is known as Deep-Stall.


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How are the landing, takeoff, and cruise speeds calculated for my homebuilt ultralight?
How can I calculate the takeoff, landing, and cruise speeds of my ultralight with a total weight (with load) of 120 kg, average air density of 1.23 kg/m3, wing area of 13.18 m2, and a coefficient of lift 1.15?

Do you have a Pilot's Operating Handbook (POH) for the plane? There should be charts in it to help you determine speeds.

@CrossRoads Judging by OP's previous activity on the site (as well as the description in their profile), they are building their own airplane. So no, there is almost certainly no POH, or any other data for that matter beyond what OP has been able to determine on their own.

@CrossRoads please post answers in the answer field, not in the comments.

Question should deleted, OP has another post with same question where it is further noted he is building a plane from scratch with data, such as POH, available.

@CrossRoads I see no such post. Please link.

I have no answer. OP is building his own plane per his profile "I am currently working on a homebuilt ultralight" and see his other questions regarding 1 vs 2 engines, moving the control surfaces, etc.  There is no POH for looking up speeds such as might be found in a kit from a manufacturer.

For constant weight, lift coefficient and air density, the resulting speed will be constant, too...

Please add whether you have a flap mechanism or not. And your CLmax value with the flap setting(s) is important.

No flap mechanism @Gurkan


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How to select the most efficient airfoil for low speed flight?
What airfoil (or family) has the highest $C_L/C_D$ ratio at speeds in the $50\:km/h$ range.
The use case would be an ultralight with a max speed of $50\:km/h$

That depends.  How low is low?  What about size?  GA aircraft? Ultralight?  Bird?  Insect?  As you get slower and slower and Reynolds number goes down, the optimal airfoil shape changes (at some point, at ultra low Reynolds numbers, the optimal airfoil is single surface; think bug wings, boat sails, hang gliders). So you need to set the size and speed range you're talking about.

@David Teahay I've edited your title to match the question in the body. Since there is no single definition of best, I hope this will save the question from being too broad.

A 5 ft chord ultralight wing at 50 km/h has a Reynolds # of about 1,400,000.  So what you need are airfoil lift and drag polars for that Re value to compare.  You might be able to find what you need here: http://airfoiltools.com/

Please read the answer to this question first and try to answer the questions listed there. You need to provide more information!
Next, the best place to look is among glider airfoils. The Wortmann FX 63-167 is now 55 years old but still a good start. It was designed for the low speed of a human powered aircraft and has been used on gliders and motor gliders.
But the airfoil still can be improved, for example by adding flaps or by narrower tolerances for the surface contour, especially around the nose. But this will make construction more complicated, so the overall best compromise might still be the Wortmann airfoil.

+1 for the Wortmann family, I used it in my end-of-studies project. Do you think the cusp on the trailing edge is worth mentioning at all? It complicates manufacturability, especially for a DIY project, which is the backdrop of OP's questions.

@AEhere: You are right about manufacturability, and I linked to another answer which proposes the S3410 in the comments for exactly that reason. However, if you want the best performance at low speed, there is no way around more camber and a concave bottom.

Thanks a billion times!.....Now I know what to do from here@Peter Kämpf

Pardon my ignorance @Peter Kämpf ,what do you mean by "narrower tolerances for the surface contour"

@DavidTeahay: Generally: Deviations from the ideal, like waviness, surface roughness, bumps in the surface. The early airfoils (like Clark Y) are more robust and tolerate more building errors. If you optimise an airfoil for one lift and one Reynolds number, the optimum will require a very high truthfulness of the actual surface contour to the calculated ideal contour.

Okay......understood!....


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Why flaps can deflect up to 25°+ without stalling the wings?
According to this graph,plain flaps have their highest Cl at about 2 and stalls at about 15°,my question is how then do airplane flaps reach 25°+ of delflection when this gragh clearly states that stall is at 15°?

The flaps being deployed 25 degrees doesn't mean your AOA is now 25 degrees... My Cessna can deploy flaps up to 40 degrees.

@Sanchises: I agree, but the answer in the other question is rather terse and deserves a more complete explanation. This is probably the reason for the new question.

@Peter I've agree (I was just trying out how short answers rather than answer-as-a-comment would be received). Regardless, they're duplicates. Perhaps close the other as a dupe of this one?

@Sanchises: Well, I voted to close this one and wrote a more complete answer for the other question.

The X-axis this is the angle of attack of the wing and not the angle of deflection of the flap. When the angle of deflection of the flap is 25%, the angle of attack of the wing will be much less.

The flaps increase the camber of the wing, not the angle of attack.
I agree the graph can be misread to mean it's the flap airfoil's angle of attack. However, from its content, it's quite clear it refers to the wing's angle - cL=2 is inconsistent with any short airfoil at a=0.


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What does "RA" mean in the FAA Terminal Constraints?
I'm trying to interpret the Terminal Constraints listed on the Operations Plan Advisory (https://www.fly.faa.gov/adv/adv_spt.jsp). RA seems to be used in the context of weather, but I haven't been able to find a definition for what "RA" means other than "Resolution Advisory" which I don't think is applicable in this context.
Example:
TERMINAL CONSTRAINTS:
NY METS/PHL/DC METS-RA/LOCIGS
CTL/ATL/MCO/TPA/FLL/MIA-RA/TSTMS

-RA is the METAR code for light rain.
LOCIGS is low ceilings, and TSTMS is thunderstorms.*

* Air Traffic Management Glossary of Terms (faa.gov).

If you can link to a reference that might be helpful for the OP

In this case, I believe the "-" is just a hyphen to separate the fields, not an indication the rain is light. All the terminal constraints use a hyphen between the airport name(s) and the conditions, e.g. "LAX-RWY 07L/25R CLOSED THROUGH 05/24/18." While -RA is light rain in a METAR/TAF context, this seems like more of an informal usage.

Could be, but I only see the hyphen used in front of those letters.

@ZachLipton No, the hypen means light. A space (or no character) means moderate and a + means heavy. This is defined precisely by ICAO annex 3 appendix 3 chapter 4 (don't remember the exact paragraph).

@Jupiter In a METAR, absolutely. But this isn't a METAR; it's an airspace planning document. Look at the current version. It says "MSP/ORD/MDW/CLT/MEM/DFW/DAL/IAH/HOU/DEN-TSTMS" Presumably that's a list of airports, a hyphen, and thunderstorms, not an indication that there are "light thunderstorms" in the vicinity of all of those airports. The next line says "PDX-RWY 1OL/28R CL0SED," and surely there's not a "light runway closure." I don't know for sure, but it doesn't seem like it means light in this context.


text:
Why does a 3D wing's CL keep increasing even after 50° AOA in XFLR5?
Can you please tell why my 3D wing in XFLR5 keeps increasing it's lift even after 50 degree AOA, while it's 2D profile stalls after about 10 degrees ?
More details on simulation setup:

Airspeed is 15 m/s (Re is about 20000-25000)
Wing chord length is 3 cm
Wing span is 12 cm
Wing is straight
Airfoil is symmetric (NACA-0009)

Thanks for your comment. What graph would you like to see ? For the 2D profile I see typical Cl drop after 8-10 degrees. With the 3D wing I refer to Cl parameter at the lower-right side of the 3D view screen.

XFLR5 is not made for this kind of calculation. Do not trust the results.
Stall is not the end of lift. See below for 2D wind tunnel data collected by S. Hörner in his book Fluid Dynamic Lift. 3D results would look similar, but with a less steep gradient around 0° and lower peaks.

180° polar for several airfoils

This is most probably due to the method you used to calculate CL_alpha. There are 3 or 4 possible ways XLFR5 support, for ex. LLT and VLM methods use potential flow based calculation without any viscous effect. If you use viscous mode which is what responsible for stall, you will see that the wing will stall ~10 or lower value depending on your wing layout.
HTH


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Are aircraft certified for inflight thrust reversal required to demonstrate controllability in an asymmetric-reverse-thrust scenario?
For most jetliners, the inflight use of reverse thrust is prohibited (and often physically impossible), due to the potential for loss of control in the event of an inflight reverser deployment (a product of the large increase in drag - and [for aircraft with wing-mounted engines], even more importantly, large decrease in lift - produced by a reversing engine).
Some aircraft, however, are certified for inflight thrust reversal (typically to allow for steeper descents by using reverse thrust as an airbrake, or to shorten the landing roll by putting out the reversers over the threshold rather than waiting until after touchdown).
Now, jet engines do occasionally fail.  As a result, one of the certification requirements for jetliners with non-centerline engines is that they have to remain fully-controllable even with one non-centerline engine shut down and the remaining engine(s) at maximum thrust.1
However, as most jetliners are not certified to use reverse thrust in flight (and most have mechanical interlocks that are supposed to physically prevent the reversers from deploying if the aircraft is airborne), they are not required to demonstrate controllability with one engine in full reverse thrust and the other(s) at full forward thrust (which, for most jetliners, would produce a thrust asymmetry around half again as great as that produced with one engine shut down and the other(s) at full forward thrust2, as well as, for aircraft with wing-mounted engines, a considerable lift asymmetry resulting from the severe airflow disruption inflicted by the reversing engine on its respective wing); indeed, these aircraft may not be controllable with one engine reversed in flight, especially at lower speeds where the aircraft's rudder and ailerons are less able to counter the large thrust asymmetry.
For aircraft that are certified for inflight thrust reversal, however, there seems to be no reason to exclude reverse thrust from consideration of inflight thrust asymmetries, as jet-engine thrust reversers, like the engines themselves, do fail from time to time.
In general, asymmetric-thrust situations involving reverse thrust would fall into two categories:

One engine is in reverse, while its twin on the other side is nonoperational; this would occur if, while reversing engines inflight, one of the engines in reverse were to fail.  This would produce a considerably-smaller thrust asymmetry than would occur with an engine in full forward thrust and its twin inoperative;2 however, for aircraft with wing-mounted engines, it would also produce a considerable lift asymmetry not present in asymmetric-thrust scenarios not involving reverse thrust, which would tend to aggravate the control difficulties posed by the thrust asymmetry.  (Aircraft with tail-mounted engines, on the other hand, would have only to contend with the thrust asymmetry, and would be expected to have little difficulty with control in such a scenario.)
One engine is in reverse, while its twin on the other side is in forward thrust; this could occur if an engine failed to go into reverse thrust when commanded but its twin reversed successfully, or if one reverser retracted without being commanded to do so, or (for aircraft with target-type reversers3) if the reverser on a reversing engine were to become physically detached in flight.  This would produce a thrust asymmetry considerably greater than that from one engine in full forward thrust and its twin shut down; aircraft with wing-mounted engines would, on top of this, also have to contend with the large lift asymmetry from having one engine in reverse and the other not.

Do aircraft that are certified for the inflight use of reverse thrust have, as a certification requirement, to demonstrate that they can still be controlled even in asymmetric-thrust scenarios involving one or more engines in reverse?

1: Actually, all civil aircraft with non-centerline engines, no matter the type of engine, have to demonstrate controllability with one engine out and the other(s) firewalled, but this question focuses on jets.
2: For most jet engines, the amount of thrust produced at full reverse is only about half of that produced at full forward thrust.  This is because:

the exhaust jet from an engine at full forward thrust is directed more-or-less straight backwards, while that from an engine in reverse is directed at large angles above, below, and to the sides of straight forwards (the exhaust jet can't go straight forwards, because the engine is in the way), and
for most high-bypass turbofans (the type used on all modern subsonic jetliners), only the bypass air (the air blown around the engine core by the fan, which accounts for most of the engine's thrust generation) is reversed; the core exhaust (the superheated air and combustion gasses coming off the turbine stages) goes straight out the back as usual, negating part (though far from all) of the reverse thrust generated by the redirected bypass air.  (Turbojets and most low-bypass turbofans, commonly used by earlier-generation jetliners, do redirect the core exhaust forwards, as this is responsible for most [or, for turbojets, all] of these engines' thrust, but no subsonic airliners have been built with turbojet engines since the late 1960s, while large-scale production of airliners powered by low-bypass turbofans ended in the first few years of the 21st century [and most manufacturers had already switched away from low-bypass engines in the 1980s and early 1990s].)

3: Target-type reversers are the big obvious ones seen on most older jetliners with low-bypass turbofans (such as the DC-9 Classic and 737 Original) as well as a few newer designs (such as the DC-9-80 and Fokker 100/70), where the big bucket doors swing out behind the engine's tailpipe to redirect the exhaust forwards.  In contrast, clamshell-type reversers (the type seen on the 727, the very first 737 Originals [before they were replaced by target-type reversers], and on most Airbus jetliners) feature doors that swing open from the sides of the engine to redirect the bypass air, and sometimes the core exhaust as well, forwards, while cascade-type reversers (seen on most high-bypass Boeing jetliners) slide the whole rear part of the cowling backwards and use cleverly-designed vanes to redirect the bypass air forwards through the gap opened up in the cowling.

Can you provide an example of a common, modern jet airliner, still in service at a reasonable level, that is certified for inflight use of reverse thrust?

@MichaelHall The C-17 can do it, though I don't know how different the certification rules are for military airplanes...

@mbrig, got a credible source showing that they use them inflight?

@MichaelHall here's a video from the RAF of it being done. Also handily explains the "why on earth": so they can have the maximum possible descent rate.


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What were/are the military designations of the JT8D and PW4000?
Turbine engines on U.S. military aircraft, in addition to their manufacturer-given names, receive separate designations (consisting of J [for turbojets], F [for turbofans; formerly TF], or T [for turboprops], followed by a number) for internal military use (examples: JT3C/J57, JT4A/J75, JT3D/TF33, JT8A/J52, JT9D/F105, PW2000/F117, JT12A/J60, CJ805-3/J79, CJ610/J85, CF700/TF37, CF34/TF34, CFM56/F108, CF6/F103), even if the military engine is completely unchanged from the civilian version known universally by the manufacturer-given name.
For most engines, information on their various names is readily available; however, I have been unable to find the military designations of two Pratt & Whitney turbofans currently-or-formerly-used on U.S. military aircraft (the JT8D low-bypass turbofan, used on the T-43, C-9, and YC-15, and the PW4000 high-bypass turbofan, used on the KC-46).  Does anyone here know the answers to this puzzle?

The reason you can't find them is that they don't exist. All US military references used the commercial designations for these engines.
Why? Because (except for the YC-15) the airframes were purchased as Commercial Off-the-Shelf (COTS) and then modified for their mission. Since the engines were already integrated into the aircraft and had an existing logistics network (documentation, suppliers, et. al.) there was no need to replicate everything within the DoD system. To do so would have been a very big expense for no real added value.
There were only 48 C-9 and 19 T-43 aircraft purchased so economy of scale was very critical in keeping costs down. And the JT-8D wasn't used in any military unique airframe.
The KC-46 is planned for 175 aircraft, but it too was designated as a COTS airframe purchase. So the engines retain the PW4062 designation. While working at a Boeing supplier providing avionics for the C-46 we had to maintain all the Type Certificated functions of the B767 while adding the military capability.
As an aside, I've worked on several products in the past that were purchased as COTS by the military. Most kept their commercial designations. A few that were significantly modified did get military component designations.
It will be interesting to see if the engines selected for the B-52 re-engining are redesignated. They might, but the goals indicate they may retain commmercial designations. From Air Force Magazine (Jan 21, 2019):

Officially, the project is called the B-52 Commercial Engine
Replacement Program (CERP), and the Air Force has already hosted a
number of industry days to dicuss it at Barksdale AFB, La., most
recently in mid-November. The stated goal: Obtain a “commercial,
off-the-shelf, in-production engine,” according to a FedBizOpps
(Federal Business Opportunities) announcement regarding the industry
day.

The YC-15 was a prototype and would not have had any logistical infrastructure set up beyond that necessary to support the flight test. The selection of the JT-8D for the YC-15 was again likely due to the cost savings as Douglas had them in their system for DC-9 production.

@ymb1 That would cover the KC-46/PW4000. The JT8D dates back to about the time MIL-STD-879 was published.

Two versions of license built military JT8D is called RM-8A and RM-8B.
PW4062 is a military version of PW4000 used in Boeing KC-46 Pegasus.

This doesn't answer the question as asked - what are the U.S. military's TF-/F- designations for the JT8D and PW4000?


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What mechanism did the turbine reversers of early high-bypass turbofans use?
Nearly all jetliners nowadays use fat, efficient high-bypass turbofans (turbofans with a bypass ratio greater than or equal to 5:1).  As the vast majority of a high-bypass turbofan’s thrust comes from the air blown past the engine core by the fan, rather than from the superheated air and exhaust gasses coming out the core’s tailpipe, thrust reversers on these engines generally only redirect the bypass air (saving on maintenance costs and reliability, as the reverser never has to come in contact with the hot, corrosive core exhaust); as such, these types of reversers (usually cascade-type reversers, where the entire rear portion of the fan cowling slides rearwards, although some smaller aircraft use clamshell-type reversers, where several discrete petals open up in the fan cowling to redirect air forwards) are often known as fan reversers (as the air they redirect has passed through only the engine’s fan, and no other rotating parts of the engine).
However, very early high-bypass turbofans (GE TF39/CF6, P&W JT9D, RR RB211), used primarily on the first generation of widebodies (747 Classic, DC-10, L-1011, A300B1/2/4), also included separate turbine reversers, which redirected the engine’s core exhaust.  Apparently, they were unreliable and required frequent maintenance (due to their frequent exposure to the core’s superheated, corrosive exhaust gasses), they were yet another complicated mechanical system on an already-very-complicated airplane, and they provided little additional stopping power on top of that provided by the fan reversers (due to the core of a high-bypass turbofan providing only a small fraction of the engine’s total thrust); as such, most airlines deactivated the turbine reversers on their fleets after a few years.1
The only reference I have handy at the moment that actually points out the turbine reversers is this diagram of the DC-10-30’s wing-mounted (#1 and #3) engines and pylons.  Unfortunately, I do not have access to a copy of the DC-10 Structural Repair Manual, so this is all I have to go on regarding its turbine reversers:

(Image by McDonnell Douglas, via the BEA [page 102, discussing a part which came loose from a different part of the CF6-50 engine, later resulting in damage to another aircraft]; modified by me.)
Other than clearing up the location of the turbine reversers (which wasn’t that much of a mystery to begin with), the diagram provides essentially nothing else in the way of information.  What sorts of mechanisms were used for the turbine reversers on these early high-bypass turbofans?  Did they use a target-type mechanism (the kind that one immediately thinks of when one hears the term “thrust reverser”, with the two big bucket doors swinging out into the airstream behind the engine), like those nearly ubiquitous on the older airliners equipped with turbojets or low-bypass turbofans?  Clamshell doors?  Something else?

1: I can think of one other potential problem; the core exhaust of a high-bypass turbofan is considerably hotter, less dense, and oxygen-poorer than the bypass air, and, thus, would tend to be more prone to causing engine surges were it to travel far enough forward to be reingested into the engine intake.  Even redirected bypass air (heated solely by the compression imparted by the engine’s fan) can easily cause an engine to surge (this is one of the reasons why the use of reverse thrust is not recommended at low speeds, the other biggie being the danger of the engine ingesting loose debris kicked up by the reverser airflow), and the much-hotter and somewhat-less-oxygenated core exhaust would presumably be even more surge-promoting were it to enter the intake; however, I do not, as of yet, have any solid evidence that this was a problem in practice, so this is pure speculation on my part (which is why it’s in a footnote, rather than the question itself).


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How exactly do the Beechcraft 1900's stabilons work?
The Beechcraft 1900, in addition to the usual wings and vertical and horizontal stabilizers, and the extra vertical tail surfaces added for improved directional stability, has a pair of horizontal tail surfaces, called "stabilons", on the sides of the rear fuselage (not to be confused with the 1900's actual horizontal stabilizers, which are mounted atop the vertical stabilizer in a T-tail configuration):

(The stabilons of these two 1900Ds are circled in green to avoid confusion with the shadows of their horizontal stabilizers on the ground.  Image by ERIC SALARD at Flickr, via russavia at Wikimedia Commons, cropped by Marc Lacoste at Wikimedia Commons, annotated by me.)
How, exactly, do the stabilons operate?  Are they fixed structures?  Do they move up and down to trim the aircraft?  Do they move along with the elevators when the pilot commands a pitch input?

Not a dupe of this question or this question, neither of which discusses how the stabilons operate.

Lol it's like a horizontal version of the multi-tails on the Connie and C-2 Greyhound.

Additionally, these aircraft have upturned winglets and... baby-sized hang-down vertical stabiliserettes (??) under the horizontal stablilisers.

This Flight Safety Training Manual describes them as improving "deep stall" characteristics and C of G range.
The stabilons are fixed surfaces that add effective stabilizer area to improve the center of gravity range, and also, by their location, add that additional surface area in a low location that mitigates the tail blanking problem you get with T tails.
So they would have just increased the area of the existing stabilizer, but that didn't fix the stall issues which are inherent to the T tail.  You could say that they added extra area as needed, and "de-T-ified" the configuration somewhat in the process by putting extra surfaces down low to provide a more positive pitch recovery than is provided by the existing T tail.  A clever application of a band-aid solution that fixed two separate problems at the same time.

For a deep stall the fuselage is too long - deep stall prone configurations have much less distance between wing and tail. But the low position of that additional tail surface is structurally advantageous, sitting at a point where its additional lift and mass can be carried by the existing structure.

The "deep stall" bit comes from the Flight Safety training manual that I linked to as my source., and I assume the writer got it from the OEM's engineering organization.  In any case made some edits.

Thanks for pointing me to the manual! On the same page they say that the T-tail puts the tail in undisturbed air - so the stabilons are in disturbed air? Reads a bit like written by marketing, not engineers. Still, they sure help to make a deep stall impossible.

Wait, I thought horizontal stabilizers on the aft end subtracted from lift, providing downforce.  Forward canards provide lift (and aren't blanked by the wings).

The stabilizer/elevator system is generally subtracting lift FOR TRIM; that is, it will provide a net downforce to oppose an overall nose down pitching moment, as part of a balance of force system that drives the entire thing to want to align itself into wind at a specific angle of attack.  For static stability purposes, this basic vertical weathervaning tendency, lift can be in either direction, depending on whether the "weathervane", the airplane's aerodynamic footprint rotating about its C of G, is on one side or the other side of its target alignment at any given instant.

@PeterKämpf: All T-tailled designs are susceptible to deep stall - ones with a longer wing-tail distance will be more susceptible, since they don't need as high of an angle of attack to blank the horizontal tail.  As for the disturbed-air bit, in normal flight, the stabilons are in the downwash of the wing (which also decreases their effective AoA and increases the nose-up trim force they provide) while the T is in undisturbed air; in a highly-stalled condition, the T is blanked out by the wings' wake, but the stabilons are in undisturbed air and provide a restoring pitch-down moment.

@Harper-ReinstateMonica: Usually yes, but horizontal tails do provide positive lift when trimmed for a CoM near the aft limit, especially at high airspeed (the range of stabilizer motion generally extends slightly into the stabilizer-nose-up zone when at near-maximum nose-down trim, and both aft CoM and high IAS tend to make a longitudinally-stable aircraft pitch up, requiring nose-down trim to counter).  Negative tailplane lift is not required; the tailplane's wing loading merely needs to be less positive than that of the main wings.

@Vikki Maybe you read this next. Having the tail in the wake is not enough - the pitching moment must have another equilibrium point, and that requires a much higher AoA than what a Beech 1900 is capable of. Also, both the lower and the upper tail are in the downwash of the wing. Please do not confuse wake with downwash.


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Can a private pilot placard an inoperative instrument?
For certificated small aircraft that are not required to have a minimum equipment list. If you have an instrument break while on a cross country flight and it is not a legally required instrument for the type of flight. You are on the ground at a distant airfield and would like to return to your home-base mechanic rather than hiring an A&P in the middle of the trip.(or if the airport has no local services)
Can the pilot legally label the instrument as inoperative for the duration of the trip without hiring an A&P to officially "deactivate" and placard the item?

In a practical sense you could simply take off and claim the instrument broke in flight. I just want to know what the "correct" answer is for the bureaucrats, officials, inspectors, oral exam, etc. 91.213 d 3(ii) mentions "if deactivation... involves maintenance, it must be... in accordance with part 43" but to me that is extremely vague, is breaking not deactivation? Does breakage not imply that it will eventually involve maintenance?

Not sure if it's per regulation, but one training flight we noticed that one of the fuel gauges was stuck (it didn't move even though we were draining from both tanks, we confirmed that after landing). The instructor wrote it up in the aircraft log, notified the maintenance department, and the scheduling department to have it looked at before the next flight. The instrument wasn't fitted with a sticker (whether that's because he didn't have one with him or what I don't know).

But you were in flight at the time the failure was noticed and landed at your base with the preferred maintenance department. Yes? This is basically the opposite of what my question is about.

Yes, we noticed in flight that one of the fuel gauges wasn't moving. It was written up, the aircraft continued operating that day, and the next time I went up a week later it'd been fixed.

The answer to your question is probably. The rule governing this is

§91.213   Inoperative instruments and equipment. (a) Except as provided in paragraph (d) of this section, no person may take off an
  aircraft with inoperative instruments or equipment installed unless
  the following conditions are met:

and if you skip past the MEL section (which, as you noted, doesn’t apply to most small general aviation aircraft) you see the conditions that must be met.

(3) The inoperative instruments and equipment are—
(i) Removed from the aircraft, the cockpit control placarded, and the
  maintenance recorded in accordance with §43.9 of this chapter; or
(ii) Deactivated and placarded “Inoperative.” If deactivation of the
  inoperative instrument or equipment involves maintenance, it must be
  accomplished and recorded in accordance with part 43 of this chapter;
  and
(4) A determination is made by a pilot, who is certificated and
  appropriately rated under part 61 of this chapter, or by a person, who
  is certificated and appropriately rated to perform maintenance on the
  aircraft, that the inoperative instrument or equipment does not
  constitute a hazard to the aircraft.

Suppose your landing light burns out. A landing light is not required unless you are flying for hire, so you can put a piece of tape under the switch and label it Inoperative and you are good to go.
If your transponder fails, turn it off and mark it Inoperative. If you are going to fly in airspace that requires a transponder, you have to jump through some hoops per §91.215, but you are still good.
If there is something that requires removal or disabling—and it is not something you are allowed to do in Part 43—then you need an A&P to remove or disable it and make a logbook entry. I’m having a hard time thinking of something that would need disabled and yet you could still legally fly the airplane. The best I can come up with right now is a stuck push-to-talk button. That’s not listed in Part 43 as something that you can perform maintenance on, so an A&P would have to disable it and make a logbook entry.
By the way, you can fly forever with inoperative equipment. There is a LORAN in plane I fly that has been marked inop since they decommissioned LORAN. When I put a new radio in my plane 10 years ago, I kept the old one in case something happened to the new one, and marked the nav portion inop.

Don't forget that some equipment listed in the AFM as "required" must be functional for flight, even if it's not listed in 91.215. I seem to remember the C172 "required" a pilot's seat cushion, for example. :-)  Since all that stuff in the AFM is incorporated into the type certificate for the craft, it has to be functional. And of course, there's common sense and safety, regardless of whatever anyone else says.

@grumpy1arrival If you want to make a flight with inoperative required equipment, it is sometimes possible with a ferry permit. I don’t know exactly how that works though.

Indeed. In the US, a ferry permit can be obtained by contacting the local FSDO. (Just noting this here in case someone stumbles upon this thread someday.) Described here: https://aviation.stackexchange.com/questions/814/how-do-i-obtain-permission-to-fly-an-airplane-that-isnt-airworthy

On further review and [informally] consulting with somebody from the FAA on the interpretation. The key answer is in "(4) A determination is made by a pilot, who is certificated and appropriately rated under part 61 of this chapter..." Basically this gives the pilot very broad authority regarding placarding and the whether their is a need to have an A&P disable the item.

For the US, 14 CFR Part 43.3(g) says:
“[T]he holder of a pilot certificate issued under part 61 may perform preventive maintenance on any aircraft owned or operated by that pilot which is not used under part 121, 129, or 135 of this chapter.”
Appendix A of Part 43 contains the specific list of things a pilot can do. In general, if it's not on that list, a private pilot without A&P certification cannot perform the task.
At the time of this posting, it reads:

(c) Preventive maintenance. Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:
(1) Removal, installation, and repair of landing gear tires.
(2) Replacing elastic shock absorber cords on landing gear.
(3) Servicing landing gear shock struts by adding oil, air, or both.
(4) Servicing landing gear wheel bearings, such as cleaning and greasing.
(5) Replacing defective safety wiring or cotter keys.
(6) Lubrication not requiring disassembly other than removal of nonstructural items such as cover plates, cowlings, and fairings.
(7) Making simple fabric patches not requiring rib stitching or the removal of structural parts or control surfaces. In the case of balloons, the making of small fabric repairs to envelopes (as defined in, and in accordance with, the balloon manufacturers' instructions) not requiring load tape repair or replacement.
(8) Replenishing hydraulic fluid in the hydraulic reservoir.
(9) Refinishing decorative coating of fuselage, balloon baskets, wings tail group surfaces (excluding balanced control surfaces), fairings, cowlings, landing gear, cabin, or cockpit interior when removal or disassembly of any primary structure or operating system is not required.
(10) Applying preservative or protective material to components where no disassembly of any primary structure or operating system is involved and where such coating is not prohibited or is not contrary to good practices.
(11) Repairing upholstery and decorative furnishings of the cabin, cockpit, or balloon basket interior when the repairing does not require disassembly of any primary structure or operating system or interfere with an operating system or affect the primary structure of the aircraft.
(12) Making small simple repairs to fairings, nonstructural cover plates, cowlings, and small patches and reinforcements not changing the contour so as to interfere with proper air flow.
(13) Replacing side windows where that work does not interfere with the structure or any operating system such as controls, electrical equipment, etc.
(14) Replacing safety belts.
(15) Replacing seats or seat parts with replacement parts approved for the aircraft, not involving disassembly of any primary structure or operating system.
(16) Trouble shooting and repairing broken circuits in landing light wiring circuits.
(17) Replacing bulbs, reflectors, and lenses of position and landing lights.
(18) Replacing wheels and skis where no weight and balance computation is involved.
(19) Replacing any cowling not requiring removal of the propeller or disconnection of flight controls.
(20) Replacing or cleaning spark plugs and setting of spark plug gap clearance.
(21) Replacing any hose connection except hydraulic connections.
(22) Replacing prefabricated fuel lines.
(23) Cleaning or replacing fuel and oil strainers or filter elements.
(24) Replacing and servicing batteries.
(25) Cleaning of balloon burner pilot and main nozzles in accordance with the balloon manufacturer's instructions.
(26) Replacement or adjustment of nonstructural standard fasteners incidental to operations.
(27) The interchange of balloon baskets and burners on envelopes when the basket or burner is designated as interchangeable in the balloon type certificate data and the baskets and burners are specifically designed for quick removal and installation.
(28) The installations of anti-misfueling devices to reduce the diameter of fuel tank filler openings provided the specific device has been made a part of the aircraft type certificiate data by the aircraft manufacturer, the aircraft manufacturer has provided FAA-approved instructions for installation of the specific device, and installation does not involve the disassembly of the existing tank filler opening.
(29) Removing, checking, and replacing magnetic chip detectors.
(30) The inspection and maintenance tasks prescribed and specifically identified as preventive maintenance in a primary category aircraft type certificate or supplemental type certificate holder's approved special inspection and preventive maintenance program when accomplished on a primary category aircraft provided:

(i) They are performed by the holder of at least a private pilot certificate issued under part 61 who is the registered owner (including co-owners) of the affected aircraft and who holds a certificate of competency for the affected aircraft (1) issued by a school approved under §147.21(e) of this chapter; (2) issued by the holder of the production certificate for that primary category aircraft that has a special training program approved under §21.24 of this subchapter; or (3) issued by another entity that has a course approved by the Administrator; and
(ii) The inspections and maintenance tasks are performed in accordance with instructions contained by the special inspection and preventive maintenance program approved as part of the aircraft's type design or supplemental type design.

(31) Removing and replacing self-contained, front instrument panel-mounted navigation and communication devices that employ tray-mounted connectors that connect the unit when the unit is installed into the instrument panel, (excluding automatic flight control systems, transponders, and microwave frequency distance measuring equipment (DME)). The approved unit must be designed to be readily and repeatedly removed and replaced, and pertinent instructions must be provided. Prior to the unit's intended use, and operational check must be performed in accordance with the applicable sections of part 91 of this chapter.

Don't forget also, of course, that "required" equipment involves not only the Part 91 required items for the type of flight, but also what is written in the AFM, STC documents and any placards that might apply to the intended flight conditions.

This would be an excellent answer to whether a PPL could repair the broken instrument, but I don't see how it addresses merely adding an "INOP" sticker to said instrument without actually repairing it. Does that even qualify as "maintenance"?

Right, but we already know that we can't just placard it... per the FAR, it has to be both marked INOP and "deactivated" (the definition of which is supposedly intentionally vague). So if that is the question, it's definitely "No".

If the instrument is non-functional, then it deactivated itself and no maintenance is required--just the INOP sticker. If it is malfunctioning, then in some cases you could pull it's breaker to deactivate it, which is also not maintenance. What non-required equipment truly requires "maintenance" to deactivate? For safety reasons, a pilot should be able to shut off anything non-essential in flight, e.g. to reduce battery load if the alternator fails.

I suppose this touches my question; changing fuses is maintenance, even vacuuming the interior and washing the windows is maintenance. Where I come from disabling an item is not maintenance. (because the state of the widget is not maintained)

Keeping in mind my question is completely academic. The practical answer would be formed based on the needs of a specific situation and all the fine details of real life.


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What is the initial direction from Sydney to Santiago on a great circle?
When flying the great circle route from Sydney to Santiago, what direction do you head leaving Sydney? Does the path leave Sydney heading SE and approach Santiago strictly from the SW, like is shows in the graphic from this answer?

Yes, the map is accurate. You can see the same thing on this map from gcmap.com:

On a polar orthographic projection, the great circle looks like this:


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Is the compressibility of air the reason high aspect ratio wings are more effecient
As stated many times here 1, 2, 3 wings generate lift by moving air downwards. This downward air creates an equal and opposite force pushing the aircraft up.
High aspect ratio wings have a greater lift/drag ratio because they are moving large amounts of air a little bit, vs lower aspect ratio wings which move a little bit of air a lot.
Now my question is why does this matter?
When using "solid" objects to generate "lift" this doesn't matter as long as the solid object is strong enough not to break. For example the image below shows 2 different sized poles (different aspect ratios) resisting the same amount of force.

Air is not a solid, and so when high pressure is exerted against it, air will compress absorbing some of that pressure and converting it into heat and potential energy.
There is also pockets of low pressure that will form which can try to "suck" the aircaft back into it creating drag. In some cases these low pressure pockets cause the water vapor in the air to even evaporate:

Is this the reason it's more effective to move a lot of air slowly, vs a little bit of air a lot? To prevent the energy from being lost to heat conversion/ storing potential energy that your aircraft will not beneficent from as you already flew out of the area of the release?
If that is true would the same apply to water which is a non compressible liquid? Is this why we don't often see boats with "high aspect ratio" displacements of water?

Regarding the comparision to "high aspect ratio" boats, I submit that they do exist.  Case in point: Rowing sculls, very long and narrow.  (However, not for the same reasons.)  Look up "hull speed" in Wikipedia for a better explanation than I am prepared to give.

Wouldn't a boat that's very long and narrow be comparable to a plane that has a low aspect ratio (short wings longer body) design? I'll look up hull speed as you suggested

Comparable, maybe... that's why I mentioned it.  However, very different as well.  Read up on hull speed, it is interesting if you are into this sort of thing.  Cheers.

Correction: the water vapor condenses to form contrails or "clouds" around the aircraft (as in your picture), it doesn't evaporate. It's already evaporated, that's why it's water vapor in the first place.

No. In fact, compressibility does not really come into play until you come into ground effect or high Mach numbers.
Let's consider placing an object (why don't you try your (empty) coffee cup?) in mid-air. The moment you release it, it will rapidly accelerate downwards and eventually disassemble spontaneously. This is because air can move out of the way of the coffee cup. The only reason the cup experiences some resistance is due to the viscosity in air (very minor effect) and the inertia of the air that's in the way.
Lift generation relies almost solely on the inertia of the air. You accelerate air downwards, and due to the inertia of the air mass, you feel a force opposite of this, that is, upwards.
The reason high AR wings are more efficient at this process has to do with reducing induced drag. Specifically, it is more efficient to accelerate a large mass of air a little bit, than a little bit of air a lot. Copying from an earlier answer of mine,

The force to keep the object aloft is $$F=m_{object}g$$ The force generated by downwards momentum transfer is $$F=\dot{m}v$$ with $\dot{m}$ indicating mass flow (kilogram per second) of the air (not the mass of the object). The energy flow (power) required to  impart this momentum on the airflow is $$ P=\frac{1}{2}\dot{m}v^2$$ Here we can draw an important conclusion. The power requirement is arbitrarily small, by increasing the mass flow and decreasing the downwards velocity.

This is the main reason for high AR wings: by increasing the wingspan (so, for equivalent surface, we increase the AR), we increase the amount of mass flow $\dot{m}$ affected by the wing.


text:
Why is there this open gap (?) above the root of this DC-10's horizontal stabilizer?
Aircraft with trimmable horizontal stabilizers need some way of sealing the area around the root of the stabilizer, so it can move up and down for pitch trim without opening up gaps in the fuselage that would admit weather and debris and create extra drag.
...Except, it seems, for this DC-10, which appears to have a pronounced open gap in the side of the fuselage just above the stabilizer root:

(Image originally by clipperarctic at Flickr, via Russavia at Wikimedia Commons; cropped and annotated by me.)
What's going on here?

I suppose the thing to note is that the area behind the rear door is home to the rear pressure bulkhead, the area behind that is unpressurized, so having a hole there (as long as it doesn't introduce a lot of drag) really isn't a problem.


text:
What is the difference between the series and parallel yaw-damper systems for the 707?
Some excerpts from the Pan Am 707 AOM indicate that the 707 can have one of two different yaw-damper systems, "series" and "parallel", with differing capabilities:

YAW DAMPER
The series yaw damper is always ON.  On parallel yaw damper airplanes the yaw damper should be turned on when comfortably airborne (1000 ft. suggested).  Remember to trim and release rudder pedal forces before engaging so that the damper has the proper neutral point.

FIRE
[...]
PHASE II

Autopilot/Yaw Damper - OFF
(Leave Series Yaw Damper - ON)  Since rudder trim will be necessary, disengage parallel yaw damper/auto-pilot until any trimming of the aircraft is done.  The aircraft can be trimmed with the series yaw damper ON.  After the aircraft has been trimmed, the parallel yaw damper and the autopilot may be re-engaged. [...]

An FAA notice sent out following a 1969 707 crash also mentions this difference:

[...] EITHER OF THE FOLLOWING PILOT ACTIONS WILL IMMEDIATELY RELEASE THE TAB LOCK LEVER WHICH MECHANICALLY LINKS THE CONTROL TAB TO THE RUDDER PEDALS FOR MANUAL RUDDER OPERATION:

TURNING OFF THE RUDDER POWER SWITCH.

RETURN RUDDER PEDALS TO A NEUTRAL POSITION.
NOTE: CAUTION MUST BE USED IN TURNING OFF THE RUDDER POWER SWITCH WITH THE RUDDER AT OR NEAR FULL DEFLECTION TO PREVENT A RAPID CHANGE IN RUDDER POSITION WHICH MAY ADVERSELY AFFECT AIRCRAFT CONTROL.
NOTE: THESE CHARACTERISTICS APPLY ONLY TO AIRCRAFT HAVING THE SERIES YAW DAMPER. [FAA Notice 8430.115, reproduced in NTSB/AAR-70/08, Appendix G, p. iii (page 56 of the PDF); my emphasis.]

What is the difference between the series and parallel yaw-damper systems, and why do they have differing operational limitations?

A serial system takes inputs, generates outputs, and there is no feedback to the input. A parallel system is a subset of a larger system, where the parallel system outputs also change the inputs. Similar to the inner/outer-loop story: a serial flight control system does its job without the pilot noticing any changed behaviour on the flight controls, while a parallel system does move the flight control, or changes the force.
The serial yaw damper would do its job, wit the only noticeable result being that the aeroplane behaved in a stable manner in the yaw axis. The parallel one would move the pedal trim position while operating.
When turning OFF rudder power while the serial yaw damper is still commanding a rudder deflection (unbeknownst to the pilot, since they are not aware of its output), there will be a large jolt on the pedals when they are suddenly loaded with airforces, resulting in a surge in yaw deflection.

That is not what "series" and "parallel" usually mean; going by the normal definitions of the two terms, one would expect a parallel system to simply have a pair of yaw dampers acting in parallel (presumably for redundancy).

Yes then the yaw dampers are parallel to each other, and will be fighting each other when one has failed. The serial damper will always be at he output end of the preceeding  system, most likely a hydraulic actuator.


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Why didn't Boeing produce its own regional jet?
Boeing, although it produced many larger narrowbodies, never designed its own regional jet; the only regional jet it ever produced was the DC-9 (from 1997 through 2006, in the DC-9-80 and DC-9-90 series), a (McDonnell) Douglas aircraft which Boeing inherited from Long Beach through a corporate merger.
Why didn't Boeing design and build its own regional jet earlier, to compete with the DC-9?

This is just a guess, but seeing as the first DC-9 was produced in 1965 and the first 737 was produced in 1966, it would be unlikely that Boeing would design a slightly smaller plane in parallel with the 737 and possibly compete with themselves.

The 737 was basically created to be Boeing's regional jet, but ended up growing into something larger to meet customer demand.
The DC-9 was introduced in 1965 and had variants seating from 90 to 135 in a single class. The 727-100 was introduced around the same time and already covered the upper end of this range. The 727-200 was even larger, almost comparable to a modern 737, so it wasn't really a "regional jet" as we would describe it today, and didn't compete directly with the DC-9. Boeing was looking for something to supplement the larger 727 and better cover lower capacities.
The 737-100 was introduced in 1968. It was designed to be low enough to the ground to allow built-in air stairs and for ground crew to be able to load/unload baggage without equipment. The capacity of 103-118 in a single class placed it right in the middle of the DC-9 market, but the DC-9 and other competition already had a head start. Only 30 of these were built, mostly for Lufthansa. The 737-200 was introduced to meet customer demand for something larger, seating 115 to 130 in a single class. This covers the upper range of the DC-9 family, and over 1,000 of these were built, surpassing the DC-9. These variants were even powered by some of the same variants of the Pratt & Whitney JT8D that the DC-9 used. It carried slightly fewer passengers than a 727-100 but was much lighter and with only 2 engines would have been cheaper to operate. With a gravel kit it could operate even on unpaved runways, and a few examples are still flying for this reason.
The 737-500 was the Classic version that was introduced in 1987 to replace the 737-200, and almost 400 of these were built. The 737-600 was the Next Generation version to replace the 737-500. The 737 had grown to optimally carry more passengers, and only 69 were built. The last 737-600 was built in 2006, with some orders being converted to the slightly larger 737-700. The smallest 737 MAX is the -7, which is slightly larger than the -700, and has not sold very well.
While the 737 NG was ramping up, Boeing did produce what began as the McDonnell Douglas MD-95 under the Boeing 717 name, but production ended amid slow sales. When the market for this size of plane picked up again, Boeing decided not to compete with companies such as Bombardier and Embraer in the regional jet market, as that would probably require a completely new design.

Sounds like you're saying the 727 is not an RJ.

Yeah, that's what I thought too.

Boeing did have a small regional jet called the Boeing 727. This plane was designed to operate at smaller airports, with independence from ground facilities as a selling feature. The best example is that the 727 had built-in stairs in the rear underbelly of the aircraft. This could be opened in flight until some hijacker known as DB Cooper jumped out to make his escape.
The 727-100C could have a combination of 54 passengers and  four cargo pallets of freight or up to 94 passengers in a mixed class with no freight.
Until it ended production at 1,832 aircraft, the 727 was the most popular passenger jet ever sold. Since that time, the most popular plane ever sold was the Boeing 737, which could have as few as 85 passengers with the 737-100.
Boeing met a definite need for airlines since it sold so many aircraft. A better question might be what could McDonnell-Douglas have done to meet airline needs and still be a viable competitor.
Two Class Seating Arrangement:

https://frequentlyflying.boardingarea.com/vintage-airline-seat-map-american-airlines-boeing-727-100-from-1977/

After outgrowing the Dassault Falcon, Federal Express built their business off of the domestically-operated B727. It was a great plane for what it was used for. But I would imagine that the technology that made smaller jet aircraft cheaper to operate than the existing turbo-prop regional carriers just wasn't quite advanced enough yet.

The DB Cooper switch was a pivoting vane (like the AoA sensor) which blocked the aft stair from coming down in the slipstream.  It was retrofitted.

The 737-100 also had built-in stairs. You can draw a story arc from that feature to the death of 346 people on the 737 MAX: How a 50-year-old design came back to haunt Boeing with its troubled 737 Max jet.

I'm not sure the 727, which seated between 150 and 180 people in a 3+3 configuration is really what would be called a regional jet today.

To add to @DavidRicherby, the 727 had a range of 2,250-2500 nm. Compare that to the range of modern RJs, at around 1500 nm.

The numbers on the wiki articles you linked don't match the ones in the answer. Wiki says a 2-cabin 727 would have 106 seats for the -100 and 134 for the -200. For single-cabin configs, the numbers are 131 and 189. That's much more in-line with a 737 or A320-size aircraft than a regional jet.

@reirab United, Eastern and American saw the 727 as a regional plane in the early 1960s. I added the seating arrangement for an American Airlines 727-100 to help eliminate your confusion. My seating numbers came from the 727-100C which could carry both freight, passengers and had built-in stairs, making it truly a regional airline.

@DavidRicherby Perhaps what's considered a "regional jet" today simply wasn't needed because using the larger 727 was equally or more feasible at the time. Less diversity in the fleet might mean lower maintenance costs, more flexibility in planning, etc. that paid off for the airline. So the question then becomes why an even smaller regional jet has a market today.

@jpmc26 The modern regional jet wasn't new at all, it was fundamentally an upgrade from prop to jet. The RJs replaced turboprops like the Saab 340, Embraer 120, and before that, Convair 340, Fokker 27, etc. (In fact, the ERJ is a EMB120 they stretched and put jet engines on)

@user71659: And swept the wings, too (although the original ERJ design did indeed call for straight, EMB-120-style wings).

Well it did have one in the form of the ‘slugs’ - the earlier 737 classic aircraft.  They sold outrageously well along with the 727s to fill regional or national routes.
And let’s not forget the airline business was considerably different than it is today in the form of structure and operations so what we consider a ‘regional’ aircraft is very different to what it was back then.
It really wasn’t until the 1990s with deregulation in full force and changes to the labor structure of the airlines going over to the ‘hub and spoke’ approach to travel, which facilitated the rise of the regional carriers and the intro of the fast and efficient 50-60 passenger regional jets.  I suspect back in the ‘golden age’ of air travel pre deregulation and oil embargo, when 747s were a status symbol and iconic of the glamour of jet setting, there just would not have been a market, an interest, or both, in a large enough setting to justify investing in a jet that small.

It's worth noting that "changes to the labor structure of the airlines" also included scope clauses in union contracts, dictating the number and size of aircraft that could be flown by contracted regional carriers. This set the demand for 50 seat (and later 70 and 76 seat) aircraft and created incentives to prefer them over larger models.

I'd argue the contrary: the cheap oil of the late 80s-90s brought about the 50 seater RJ. They are definitely not efficient compared to the turboprops they replaced and 100-seaters. They've effectively stopped making them for this reason: the CRJ200 and ERJ145 are out of production. (More than a decade for the CR2)

Via a third party

Boeing is heavily involved in design and marketing this beauty, the Sukhoi Superjet 100. 78 or 98 seats, with talk of a stretch or two to 120 and 140.  Russian built, with largely Western engines, subsystems and avionics.  It was poised to do quite well in the RJ market until Euromaidan/Crimea/Donbass threw a big monkey wrench in the political situation.  The aircraft is also having the teething pains one expects from a blank-sheet design, and had one all-souls loss entirely not the aircraft or engines' fault.  Still, the Superjet soldiers on, and it is closing some sales, with 300 orders on the books.  Pretty spectacular for a Russian aircraft, and that's because of Boeing.

The 737 started as an RJ
The famous and popular -800, -900, -8 and -9 stretch so far beyond original design concepts that they actually have tail-strike risks.

The 737-100 seated 85 passengers and was 94' long.
The 737-200 (original), -500 (Classic), and -600 (NG) are all ~102' long and seat about 100.  This size has not been continued in the 737 Max, so this is Boeing abandoning the "RJ" 737.   Nonetheless, Boeing has the designs, and I am sure Boeing cheerfully sell -600s or even "Max 6"s if enough airlines wanted an RJ co-qualified with their bigger 737s.  So far, they do not.  Nor is it likely, given what a poor seller the A318 has been, and even the Max 7.

MD-95 / "717"
Boeing inherited the DC-9 as part of the McDonnell-Douglas merger (which was mostly about military aircraft). This could have been their RJ, but they failed to continue developing it.  though, so we don't have a "717 Max 4" or anything (couple of GE Passports, wouldn't that be nice!)
Boeing also passed on opportunities to gain an RJ via acqusition of Fokker or Canadair. The Sukhoi toe-dip is as close as they've gotten.

Other answers have noted the history of both the 727 and 737, which started as small regional jets, but grew due to demands from established customers. Regional jets, by contrast, tend to be flown by smaller, regional airlines that transitioned from turboprops (such as the the Bombardier Dash 8) and consequently went to the same manufacturer when they wanted to transition to a regional jet.
To directly answer the question, Boeing apparently did not develop their own regional jet in-house because:

They believed it would detract from the development of other aircraft that had more profit potential (the 737-MAX and 787 in recent years), and
Their history with smaller 727's and 737's and the inherited 717 indicated there was an insufficient market for such aircraft.

Instead, when it was finally apparent to them that they needed a regional jet, they elected to make a buy-versus-build decision and entered into a majority stock purchase of Embraer (after the Brazilian government made it clear that an outright acquisition was out of the question). This keeps the development isolated, hence addressing the first concern, and limits their exposure, hence addressing the second concern.


text:
Have bi-wings been used without creating more drag?
As I understand from answers I've read here 1, 2, 3, biplanes create more lift with less wing span when compared to single wing aircraft, because the combined wings have greater surface area overall.
However this is usually at the expense of added drag as both wings are accelerating the same air downwards causing interference and creating inefficiencies. Also the wings commonly have supporting wires/structures between them causing more drag.
With that said biplanes do have some desirable characteristics like shorter roll response. I also realize when looking at how nasa calculates for the drag coefficient, wing aspect ratio is a major factor in their equation:

With this information it would seem to me if you could separate the wings of a biplane far enough apart that they don't cause much interference, you could possibly get a better performing aircraft with the same wingspan.
I almost imagine it being as simple as making the tail a full sized wing and giving each wing a thinner aspect ratio. However I've never seen this done before or even attempted. Is there a reason for this? Would the wings still interfere too much with each-other.

Frankly as far as I can tell, the disadvantages of biplanes greatly outweigh the advantages you'd get through increased lift i.e. surface drag, unnecessary weight gains, etc. Biplanes just aren't efficient enough in the end game for most all applications. There are a few where an optimized design perhaps like your idea where it might prove useful, but in the mainstream no I don't think so.

And yes also having a wing in the tail would not only offset the CoL too much, but it would almost definitely receive interference and turbulent flow from the front wing, unless it's mounted low/high enough too where a whole new series of issues would arise. Sorry but it's just too much trouble to implement radical designs.

Moving the wings vertically apart would mean more and heavier supporting structure, but also a higher pitch moment of inertia. Since this grows with the square of the vertical expansion, a biplane with a lot of distance between both wings will be hard to control and very sluggish.
The real advantage of a biplane is reduced induced drag because less structure needs to be lifted. When cross-bracing enables the top wing to work as the compression member and the bottom wing to work as the tension member of a truss, a very lightweight structure becomes possible, especially when the size of the aircraft grows.
If all you have to power your airplane is a heavy, inefficient piston engine like those at the beginning of the last century, and you can tolerate that your aircraft won't fly fast or far, a biplane is indeed the lowest-drag solution.

Your answers always explain things from an angle I've never considered. Now I have to look up what effects pitch inertia have. Thank you!

" a biplane with a lot of distance between both wings will be hard to control and very sluggish." - I disagree. Pitch resistance due to moving the wings apart would be nominal, both because they represent only a tiny amount of the pitch mass (they are both lightweight and located near the center of rotation) and because they are moving in the direction of the relative wind. The engine, on the other hand, is both heavy and a long way from the COR. Contrast this with roll resistance, where the engine is now at the COR and the wing surface is moving against the relative wing.

@MauryMarkowitz: The question is about moving them apart. And then they are not close to the center of rotation.

How does the lower wing act like the tension member of a truss,I understand how the upper wing is a compression member(because the flying wire attached to it is at an angle) but what I don't get is how the lower wing is a tension member?...pls can u explain just a little bit @Peter Kämpf

As for "making the tail a full sized wing", this is a reasonable idea and it has been tried on quite a few designs. Such configuration is called tandem wing. Perhaps the best known example is Quickie:

Wikimedia
Purely aerodynamically, tandem is better than biplane, due to both the wing-wing interference and the lack of horizontal tail. However, tandem doesn't have the structural benefits of the biplane (see Peter's answer) and so must be heavier. In which conditions it may be preferential to the classical design deserves a separate analysis, but given the scarcity of such airplanes, they must be quite narrow.

Wow I can't believe you actually found a plane like this! That's incredible thank you. I'm wondering with today's composite materials could a plane be made without the weight concerns? I'll try to look into how well this plane performed

@YAHsaves, well, there are always weight concerns: if you can make this design lighter, you can potentially make biplane even lighter. That said, there are many other considerations (for example, wall thickness: you can't make it too thin), and the composites do shift the balance towards clean console and monocoque designs. It's not an accident that Quickie is a composite airplane (even though it's from the '70s).

Your question of "better performance" can be yes or a no, depending on what you're after. Engineering is a management of compromises. Understanding the pros and cons of a biplane design will prove that the disadvantages usually outweigh the advantages, but perhaps not- again it depends on what you're after.
Engineers have learned things over the last 100+ years- Stacked wings will not generate twice the lift of a single wing, their aerodynamics interfere with each other BUT you will absolutely get more lift than a single wing of the same span.
Remember wingtip vortices kill lift, and you'll have four wingtips. Stacked wings always add drag. If you have two cantilever wings you could reduce the drag from whatever struts and wires you eliminate over the older designs, but it will still have more drag of all sorts than a monoplane. You might like simpler wing design as biplanes have greater STOL performance without flaps, The compromise here- you'll have a tougher time handling that much wing in a crosswind.
Flaps give you flexibility of higher lift and a lower stall without so much wing area. Plus you'll really need to slam the stick forward if you have an engine failure, these planes really slow down fast.
Back in the day biplanes were hard to beat. The first monoplanes had just as many wires, the first cantilever wings were poorly made and killed a few people, which made pilots deathly afraid of them. Change is hard to push onto people when the tried and true works so well like good lift and high strength, but the solutions are so well understood today the biplane's only advantages are maybe some margin of higher strength, Easy inexpensive strength, greater lift in a given span, short wings roll faster and an awesome look and experience.
Novelty can be fun, and you do gain some things with trade offs. I'd say don't quit if you like the idea, do it. They certainly do fly and can fly well, and you could improve on it. If you just want to fly Cable trussed biplanes give you inexpensive, easier to design strength. You might have a lot of fun- but you won't improve fuel economy, safer ground handling, speed or a longer glide.

@David Bloxham, Thank you for your response. That's an interesting point to think about high cross winds when on the ground if you have no way to offset the lift. Regarding wing tipped vertices, I actually found out recently from this site they don't cause drag directly, but are simply part of the lift process. You can read about this here: https://aviation.stackexchange.com/questions/31996/why-does-the-elliptical-wing-have-the-lowest-drag/31998#31998


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Why does the DC-8-70 have the same MTOW as the DC-8-60, despite its more powerful engines?
The DC-8-70 is a more-or-less straight reengine of the DC-8-601, replacing the obsolete JT3D-3B low-bypass turbofans of the DC-8-60 with the CFM56-2C1 high-bypass turbofan.  According to Wikipedia, the maximum takeoff weight of the DC-8-70 was unchanged from that of the DC-8-60 (which resulted in the DC-8-70, with the heavier CFM56, having a slightly lower payload capacity than the DC-8-60):

The DC-8-71, DC-8-72 and DC-8-73 were straightforward conversions of the -61, -62 and -63 primarily involving replacement of the JT3D engines with more fuel-efficient 22,000 lb (98.5 kN) CFM56-2 high-bypass turbofans with new nacelles and pylons built by Grumman Aerospace and fairing of the air intakes below the nose. The DC-8-71 achieved the same end but required more modification because the -61 did not have the improved wings and relocated engines of the -62 and -63. Maximum takeoff weights remained the same, but there was a slight reduction in payload because of the heavier engines. All three models were certified in 1982 and a total of 110 60-Series DC-8s were converted by the time the program ended in 1988. DC-8-70 conversions were overseen by Cammacorp with CFMI, McDonnell Douglas, and Grumman Aerospace as partners. Cammacorp was disbanded after the last aircraft was converted.[10]

Emphasis mine.
But maximum takeoff weight is determined by engine power,2 and the CFM56-2C1 has nearly a quarter again as much thrust (98 kN; 22 klbf) as the JT3D-3B (80 kN; 18 klbf), which should result in a proportionate increase in MTOW for the CFM56-2C1-equipped DC-8-70.
So why does the DC-8-70 have exactly the same MTOW as the DC-8-60, despite its significantly-increased engine power?

1: The DC-8-72 and -73 are literally just reengined DC-8-62s and -63s; the DC-8-61 required some additional modifications to make it into the DC-8-71.
2: It's the maximum weight at which the aircraft is physically capable of attaining liftoff speed on an infinitely-long frictionless runway at zero density altitude with one [for twinjets] or two engines [for more-than-twinjets] inoperative; beyond this weight, the aircraft has insufficient one-or-two-engines-out thrust to push it fast enough to generate enough lift to lift off.

But maximum takeoff weight is determined by engine power

not by engine power alone. The airframe also has to be strong enough for the intended takeoff weight. There is a structural strength margin, but this margin is mandated by the certification authority so you can't use it. The design won't have much more than the required margin because that would make the aircraft heavier than necessary and less competitive.
At a minimum, you'd have to get the aircraft certified at the new MTOW, which requires expensive testing. Worst case, you have to modify the design of parts like the wing box/spars, landing gear, and fuselage structure.
By 1988 the writing was on the wall for narrowbody 4-engined aircraft, with ETOPS-180 coming into force.

It's the maximum weight at which the aircraft is physically capable of attaining liftoff speed on an infinitely-long frictionless runway at zero density altitude with one [for twinjets] or two engines [for more-than-twinjets] inoperative;

No. That would make the definition of MTOW useless in the real world. Compare to the FAA definition:

The Federal Aviation Administration has defined Maximum Takeoff Weight (MTOW). The MTOW of an aircraft is the maximum weight at which the pilot of the aircraft is allowed to attempt to take off, due to structural or other limits.

The MTOW you see in manufacturer specifications is a figure established in the certification process. This will be below the physical maximum weight the aircraft can handle, to make sure there's a margin of safety.
In many circumstances (airfield runway length, altitude, temperature)

an aircraft may not be permitted to take off at its MTOW... The maximum weight at which a takeoff may be attempted, taking into account the above factors, is called the maximum permissible takeoff weight, maximum allowed takeoff weight or regulated takeoff weight.